1. Technical Field
The present invention relates to a technique of producing silicon from silica.
2. Description of the Related Art
Silicon is used in various fields such as those of semiconductors, solar cells, displays, and anode materials for secondary batteries. As the usability of silicon as an advanced material has increased recently, much of domestic and international research and development is focused on silicon production and related technologies. Silicon can be obtained from silica, i.e. silicon dioxide (SiO2). SiO2 is one of the most common compounds found on the earth, and research is being steadily made to convert and purify low value silica into high value silicon for use.
Silica as a raw material for silicon is mostly collected from minerals or sand, and, recently, research has also been actively made to use silica which constitutes the skeleton of plants. The plant-derived silica contains silica at relatively high purity due to a process of “selective absorption and fixation (natural selectivity) of a biological silica” in roots. In particular, compared to mineral silica extracted from sand, the plant-derived silica has the advantage of containing less impurities (e.g. boron and phosphorus) that affect the efficiency of solar cells. Since the impurities such as boron and phosphorus are conventionally difficult to be removed by an acid treatment process or the like, the impurities have hindered the preparation of high-purity silicon by direct reduction. However, the plant-derived silica is advantageous in that its purity can be improved by the acid treatment or the like, because in the plant-derived silica, boron, phosphorus, and the like are almost non-existent, and the content of impurities such as aluminum and iron are lower compared to other silica sources (e.g. sand).
The plant-derived silica has evolved to attain a structure capable of buffering an external impact so as to serve as an exoskeleton of plants. In addition, a silica layer of plants takes a nanoporous structure which enables water and air to pass through while protecting an internal structure of plants from bacteria, viruses, and the like trespassing from the outside. Although silica was used as absorbent, filler because of its original structure, the up-to-date research is focused on utilization of nanoporous nature of silicon for solar cell, secondary battery anode material when silica is reduced into silicon.
In reducing silica to silicon, the most frequently used method is thermal reduction (metallothermic reduction or carbothermal reduction) that uses, as a reducing agent, a metal with a large reactivity (e.g. an alkali metal) or carbon. Generally, in the thermal reduction, a reduction reaction is caused by heating to a temperature equal to or greater than the melting point (Tm) of the reducing agent. As Korean Patent No. 1405886 and Korean Laid-open Patent Application No. 2012-0033434 disclose a reduction reaction by heating to a temperature ranging from 1000 to 1200° C. in a high temperature electric furnace, an electric arc furnace, a plasma reactor, or the like, and Korean Patent No. 1527644 discloses a reduction reaction by heating to a temperature ranging from 600 to 1100° C. for 3 hours in a heating reactor, the thermal reduction always involves a high-temperature process of around 1000° C.
Even though most of the known techniques are performed at a high temperature, it is not sufficient to improve conversion, because the melting point of silica is 3000° C. or greater and the reduction reaction of silica requires very high activation energy. In addition, there are problems of the waste of resources, disposal expenses, or the like resulting from the combustion of plant fiber, volatilization by heat treatment, or decomposition by an acid treatment in acquiring the plant-derived silica. In reducing silica to silicon, there is also a problem of using a process (e.g. a high-temperature high-pressure process) that is costly relative to the supply and demand of raw materials.